The invention pertains to the art of image processing and, more particularly, to the halftoning arts. It finds particular application in conjunction with high addressable printing devices, halftone dot design and procedures for loading a halftone generator. Accordingly, it will be described with reference thereto.
The following description includes references to xe2x80x98pixels.xe2x80x99 A distinction between xe2x80x98input pixelsxe2x80x99 and xe2x80x98output pixelsxe2x80x99 is necessary at the outset for clarity. A pixel, or picture element, is generally understood to be the smallest bit of an image that a particular device can reproduce.
Image input devices, however, such as PostScript interpreters, scanners and video memory elements provide pixels or image information as an 8 bit signal per color. These 8 bit pixels typically are capable of uniquely identifying up to 256 distinct levels.
Image output devices, on the other hand, such as digital color printers, monochrome facsimile devices and the like, produce binary pixels. That is, the image output pixel is typically either a mark or lack of a mark. Groupings of these binary pixels, called xe2x80x98halftone dots,xe2x80x99 are arranged in a predetermined pattern within a digital halftone cell. In order to create a variety of output levels, groupings of output pixels are output at various frequencies. The lower the number of output pixels per unit area, the lighter the tone. The higher number of output pixels per unit area, the darker the tone.
Presently, various digital image-processing techniques are known for manipulating static images for output. Existing digital color printers, such as the Xerox(copyright) 5775 printer use digital halftone cells with about 120 levels per color. These levels are approximately uniformly distributed in reflectance space. The 8-bit input signal used to control each of these 120 output levels is assumed to be linear in reflectance. During printer calibration, a look up table is developed to map these 256 input levels into the appropriate 120 output levels within the printer space, losing a few levels in the process. This loss is not significant because the remaining input levels (about 200) are mapped onto the 120 actual printer halftone levels. Given the printer noise, 120 levels are sufficient for a medium quality output.
For higher quality output, over 150 distinguishable output levels are needed. In some printing technologies, several of the colors have a steep gamma, reaching 10% reflectance (i.e. 90% darkness) with only 50% of the input bits turned on. Even if the digital halftone cell were designed with 256 output levels, 90% of the incoming levels from the source (e.g. a scanner or Raster Image Processor) would map into the first 128 levels. Indeed, empirical testing has shown that these steep gammas combined with vagaries of human eye perception and printer rendition may not be as successful in producing the desired perception of a gradual shading change over the range of the output device. In particular, certain incremental changes in the halftone dot patterns may not be perceived as having any shading change consequence whatever. Hence, an assignment of one of the limited number of control signals (i.e. input signals) to produce an imperceptible change in darkness is effectively a waste of the control signal value.
This waste is the problem addressed and resolved by the subject invention; i.e., implementation of a system wherein all available control signal values each correspond to meaningful, distinctive halftone dot patterns. The present invention contemplates a new, efficient marking engine and method to optimize tone levels in a digital output system which overcomes the above referenced problems and others.
In accordance with the present invention, a halftone cell configuration of a plurality of output pixels has optimal tone level gradations. The halftone cell comprises output pixel patterns corresponding to the tone level gradations where selected patterns vary in form and number of output pixels from other patterns.
In accordance with another aspect of the present invention, each of the output pixel patterns corresponds to a substantially equal change in a measured darkness value.
In accordance with another aspect of the present invention, the output pixel patterns comprise a predetermined number of subpixels comprising a fractional portion of each pixel.
In accordance with another embodiment of the present invention, a method for optimizing tone level gradations in an image output device is provided. The method includes the steps of creating a tone reproduction curve defining a relationship between the number of output levels and a level of darkness. The method also includes correlating each one of the number of input levels to a corresponding one of the number of output levels based on a desired percentage change in the level of darkness.
In accordance with another aspect of the present invention, the tone reproduction curve is created by producing a calibration sample of individually measurable calibration elements representative of a plurality of the output levels. The level of darkness associated with the calibration elements is then empirically measured and plotted relative to the output levels.
In accordance with another aspect of the present invention, the correlation comprises determining a percentage change in the level of darkness between successive input levels. Where the determined percentage is substantially less than the desired percentage change, one of the input levels is unassigned from one of the successive input levels. When the determined change is substantially greater than the desired change, additional calibration elements are produced and measured until the determined change is substantially equal to the desired change.
In accordance with yet another aspect of the present invention, the halftone cell comprises outputting a plurality of neighboring halftone cells configured as a supercell.
In accordance with another embodiment of the present invention, an image output apparatus receives one of a predefined set of input levels and outputs one of a set of predefined halftone cell configurations of subpixels; such that each one of the set of input levels produces an appreciable change in darkness. The apparatus comprises a look up table preprogrammed such that one of the set of input levels corresponds to a number of subpixels, and another of the input levels corresponds to a different number of subpixels. A halftone pattern generator is included for arranging the number of subpixels into the predefined pattern.
One benefit obtained by use of the present invention is that the efficient selection of levels makes more levels available for use in steep areas of an unmodified tone reproduction curve (high gamma). This in turn reduces the possibility of visible banding.
Other benefits and advantages will become apparent to those skilled in the art upon a reading and understanding of this specification.